This invention relates to the separation of tar from aqueous solutions. More particularly, this invention relates to an apparatus, and method for monitoring and controlling the interphase level between the tar and aqueous fluid in a zone for separating the tar and aqueous fluid by settling.
In many installations, tar is separated from aqueous solutions in settling or decanter tanks. In these tanks the tar is separated by manually setting regulators which balance to the height of the regulator the weight of the aqueous phase, tar-water emulsion, and separated tar inside the tank against a weight of a column of tar isolated inside piping connected with the tank. The tar-water emulsion is formed from the contact of aqueous solution with the tar. If it is desirable to increase the removal of tar from the tank, the height of the manual regulator is lowered. If it is desirable to decrease the removal of tar from the tank, and therefore increase the quantity of tar inside the tank, the height of the manual regulator is elevated.
The problem with this manual operation is that a slight change in elevation above or below the desired level disadvantageously affects the operation. These disadvantages include: the accumulation of too much tar, thereby causing tar in the emulsion to be removed from the tank along with the aqueous fluid; or the removal of too much tar, thereby causing tar with a high moisture content, emulsion and possibly aqueous fluid to be removed from the tank with the tar. In addition, the tar in the tank has a constantly changing specific gravity because of the change in moisture content of the tar during settling. The constantly changing specific gravity of the tar with the resulting changes in tar level inside the tank makes the control of the tar flow from the tank very difficult. Frequent adjustments of the manual regulator are required to prevent the discharge of excessive quantities of tar with a high moisture content or emulsion or possibly aqueous fluid into the tar dehydrating system. The presence of emulsion or aqueous fluid in the exit tar from the decanting tank contaminates the tar. The aqueous fluid contains dissolved compounds and the presence of excess aqueous fluid in the exit tar from the decanter tank carries the dissolved compounds which decompose during distillation of the tar. This decomposition causes damage to the distillation equipment and troublesome accumulation of solids in the condensing equipment.
Many apparatus and methods are known in the art for regulating the level of fluid within a vessel. These apparatus and methods include the use of float valves, gamma ray detectors, thermal cables, wire coils and differential pressure meters. Attempts to use these apparatus in regulating the level of tar in contact with an aqueous fluid in a settling or decanting tank have not been wholly successful due to a number of factors. A recent attempt involved the use of differential pressure meters in a coal tar decanting tank. This attempt consisted of having two pneumatic dip tubes positioned vertically in the decanter tank before an initial separation of tar and aqueous fluid from the emulsion. One dip tube was positioned in the aqueous fluid and the other dip tube was positioned in the tar layer beneath the aqueous fluid layer and water-tar emulsion layer. These dip tubes were connected to pressure gauges which indicated the difference in pressure whenever the tar level rose or fell. A signal was generated from the difference in pressure and was transmitted to a control valve that increased or decreased the flow of tar from the decanter in relation to an increase or decrease in the tar level within the decanter. The apparatus and method of this attempt failed to perform satisfactorily because the presence of a large amount of tar-aqueous fluid emulsion near the dip tubes prevented the establishment of a tar-aqueous fluid interphase. This resulted in the presence of tar in the aqueous fluid or some aqueous fluid in the exit tar from the decanter.
It is an object of the apparatus and method of this invention to provide continuous automatic monitoring and control of the level of tar in a new or existing settler or decanter tank installation.